Videos are generally projected as sequences of images on a video screen by a video projection system composed of a single video projector or multiple video projectors generating adjacent, partially overlapping sub-images. Projected images can be of a standard definition or of a high definition (HD), offering high image quality.
When the video projection system used comprises multiple video projectors, each single video projector generates a sub-image with a given definition and a size determined by the video projector lens focal length, the size of the video projector's light modulation device (e.g. an LCD panel), and the distance between the video projector and the video screen.
Covering efficiently, in terms of brightness, a very large projection screen with the required definition usually requires aggregating several sub-images in a manner that they cover adjacent, partially overlapping zones of the full screen area. In the overlapping zones, blending ensures a smooth transition between sub-images projected by different video projectors in a manner that is tolerant of small displacements introduced, for example, by vibrations or thermal expansion. Blending is a well-known technique that consists in continuously decreasing the brightness of sub-images generated by a video projector towards the border of the projection zone covered by that video projector and complementarily increasing the sub-image brightness of sub-images generated by adjacent video projectors in a manner to obtain uniform brightness after superposition.
The partitioning and blending of sub-images to be projected by a video projection system comprising multiple video projectors have to be carefully adjusted to give the user the impression of a single image throughout the whole projection screen area. To that end, each video projector of a video projection system comprising multiple video projectors is set up so as to control, in particular, overlapping and blending of areas of the projected sub-images.
Such a set up typically requires exchanges of information between the projectors of the system. To that end, communication links must be established between the projectors, for example, according to a centralized scheme, between a server or master projector and each of the other projectors, typically referred to as slave projectors. Such communication links can also be used for transmitting video streams from the server or master projector to the slave projectors.
When wireless communication links are used, an initial discovery phase is generally conducted to identify radio communication paths to be used. It is noted here that for the sake of efficiency, each of the communication devices used in a wireless system, that is typically associated with each video projector, may comprise several antennas, for example several directional antennas, and/or several antenna settings, the antenna(s) being emitting antenna(s), receiving antenna(s), or mixed antenna(s). As a consequence, there commonly exist several communication paths between two communication devices.
A main issue such as an initial discovery phase has to deal with is radio interference and data collision that generally slows down the overall system setup.
Using state of the art wireless technology like CSMA-CA (acronym for Carrier-Sense Multiple Access-Collision Avoidance), discovering communication paths between projectors is subject to collision problems when using wide antennas and is longer when using directional antennas for performing transmission.
Indeed, in case of wide antenna transmission, several projectors may try to access a wireless medium simultaneously, leading to collision of the radio transmission requests. Accordingly, several access attempts are required to correctly set up a communication path.
Regarding directional antennas, discovery protocols generally require the use of a reception feedback channel. A waiting time is required for reception antenna sweeping for each directional antenna used plus additional waiting time for enabling the receiving communication devices to get access to the wireless channel and send feedback information to the source communication device. This is done without any guarantee that this time is practically used for transmitting feedback data since it may occur that a receiving communication device is not able to receive any signal from the emitting antenna of the source communication device.
There exist other methods for setting up video systems comprising several video projectors such as the one described in U.S. Pat. No. 7,942,530 that addresses, in particular, automatic identification of parts of video images each projector has to display. According to U.S. Pat. No. 7,942,530, the main video signal is transmitted to all the projectors used through a wired connection (star topology), each projector having its own camera to capture its own display and part of the surrounding area displayed by neighboring projectors. Each projector is therefore able to capture data through its camera and share items of information by projecting them as parts of images since a communication channel is initially available between communication devices, enabling setup of the system.
Accordingly, there is a need to improve the setup of video projection systems comprising multiple wireless video projectors, in particular when initial communication paths are not available.
In particular, there is a need for establishing wireless communication channels while avoiding collision disturbance and the use of a contention-based access mechanism whatever the number of slave projectors in the video system.
Furthermore, it would be desirable to enable transmission of feedback data between communication devices during discovery phases.